A key aspect in the design of water current and wave power generating systems is the method by which the power generating apparatus is periodically accessed for inspection and maintenance, and deployed again for operation. The choice of access and redeployment technique has a significant effect on the cost of maintaining the entire machine over its lifetime.
Diverse combinations of power generating apparatus, support structure and seabed foundation have been proposed for tidal current and wave devices. Machines that employ a permanently floating, surface breaking power generating apparatus do not have a significant problem in terms of initial access for inspection/maintenance.
However, machines where the power generating apparatus is fully submerged during normal operation must on occasion be brought to the surface for inspection/maintenance. Such inspection and maintenance may possibly take place at the generating site, but more likely, the power generating apparatus will be recovered to a more suitable location, such as a harbour, or dock, for inspection and maintenance. There are currently two basic schemes for such machines:                Schemes employing a surface breaking support structure (such as a mono-pile). The structure itself is used as a guide to allow the power generating apparatus to be raised to the surface using some form of mechanical lifting device (e.g. hydraulic rams, or surface mounted winch).        Schemes where the support structure lies submerged (or mostly submerged) during normal operation. Such devices may consist of:                    a power generating apparatus mounted directly to a seabed foundation;            a power generating apparatus attached by cables or long articulated arms to foundations on the seabed, wherein the level of the power generating apparatus in the water column during operation is determined by its own positive buoyancy; or            a power generating apparatus mounted on a submerged lattice or tubular framework structure that supports the power generating apparatus mid water column.                        
Water current and wave power generating devices are deployed in extreme marine environments where weather and sea state conditions can often make marine operations difficult. The speed and simplicity of the method of deployment and retrieval of the power generating apparatus, and, in particular, the tolerance of this method to moderately bad weather and sea state conditions are, therefore, important.
Conventional methods for retrieving submerged power generating apparatus from the surface include:                A crane or winch mounted on a surface vessel. The vessel required to lower/raise a heavy power generating apparatus is expensive and may not be available at short notice. The constant motion of the vessel on the surface can also pose problems for the positional control of the power generating apparatus.        A telescopic tube normally housed inside the support structure. This telescopic tube guides the negatively buoyant power generating apparatus straight up to the surface. The weight of the power generating apparatus and telescopic tube is supported by a deck mounted winch/crane on a surface vessel. This method addresses the issue of position control of the power generating apparatus, allowing the lifting operation to be performed more quickly and easily. The scheme still requires vessel mounted heavy lifting equipment, and there is a considerable overhead associated with the telescopic tube which is only used on an occasional basis. There is also the possibility of the telescopic tube jamming; with the associated risk that the machine is left in a potentially dangerous state should the weather conditions deteriorate.        Guide wire(s) running from the surface vessel down to the support structure are used to guide the power generating apparatus as it is raised through the water column. The correct tensioning of these guide wires from the support vessel requires accurate position holding and possibly heave compensation equipment, which is expensive. In the case of a water current generating machine, where there is only a short period of relatively slack water available and the recovery of the guide wires is a time consuming operation, for which there may be insufficient time.        
An existing method that relates to the deployment and retrieval of power generating equipment is specified in Gibberd et al “A deployment and retrieval apparatus for submerged power generating devices”, UK Patent No. GB2431628.
It is therefore desirable to provide a power generating apparatus, such as for example a tidal turbine assembly, and a method of retrieval of power generating equipment from an underwater assembly with improved accuracy, improved reliability and/or reduced cost considerations of retrieval.